(NBC NEWS) -- A U.S. Marine helicopter's "hard landing" near the North Korean border left 21 service members injured early Tuesday, officials said.

The CH-53E Super Stallion chopper was "conducting routine flight operations" during an exercise near South Korea's Jipo-ri Range at the time of the incident, according to a military statement.

All 21 personnel aboard were hospitalized but 15 were treated and released. Six remained in stable condition.

Its crew was from the 31st Marine Expeditionary Unit and is stationed in Okinawa, Japan.

South Korea's Yonhap News Agency reported that the aircraft caught fire following the incident, which it said occurred about 55 miles north of Seoul. NBC News could not immediately independently verify those details.

The military statement added: "Safety is a priority for all aircraft operations. The CH-53E has an excellent operational safety record. A comprehensive investigation will take place to determine the facts and circumstances surrounding this incident."

Neighboring North Korea has threatened nuclear attacks on the United States, South Korea and Japan after new U.N. sanctions were imposed in response to its latest nuclear arms test in February.

The North has also been angry about annual military exercises between U.S. and South Korean forces, describing them as a "hostile" act. The United States dispatched stealth bombers from their bases to take part.